Every cell is surrounded by a plasma membrane that divides cytoplasm from extracellular fluid and separates one cell from another. The membrane and membrane-associated components play vital roles in virtually every aspect of cell function. During embryonic development, the differentiation of a variety of histotypic cell properties is associated with gentically controlled alterations in surface-associated molecules such as receptors for hormones and neurotransmitters, transport proteins and ion conductance channels. It is the goal of this Program to understand how certain specific components of the cell membrane and the membrane-associated cytoskeleton serve as regulators of cell function, and how changes in these components with development result in the achievement of definitive functional states in excitable cells. By application of biochemical, biophysical, genetic, and ultrastructural techniques to cardiac cells in a number of in vitro model system, we hope to gain insight into the mechanisms underlying membrane function. The unifying theme of the Program is the role of cell surface components in regulating cell function. There are three major approaches. One effort focuses on specific ion conductance mechanisms in the heart cell membrane; how they develop in the embryonic heart, and what role they have in macroscopic and microscopic electrical activity. A second focus deals with receptor mechanisms for insulin and acetylcholine, and how these ligands activate currents and other cellular activities. The third area is the physiological and genetic regulation of ion transport mechanisms in the cell membrane. This is a renewal proposal for five years of support for an expanded effort by seven experienced investigators. The Program includes seven inter-related projects subserved by six core facilities.